1. Field of the Invention
The present invention relates generally to fluorescence spectometers and, more particularly, to an inductively coupled plasma atomic fluorescence spectrometer for the multielement analysis of samples.
2. The Prior Art
Until the early 1960's, general purpose atomic spectroscopy measurements had been carried out by using flame atomic emission (FAE) instruments. FAE instruments employ no external excitation source. Rather, in FAE instruments the free atoms are excited by thermal collision with high energy species of the atomization source (e.g., the flame gas atoms), and a portion of the excited atoms undergoing radiational deactivation is measured. In atomic emission (AE) spectroscopy, all of the possible energy levels above the ground state of a given element are populated and all of these excited levels are undergoing radiational deactivation. Consequently, AE spectra are spectral-line rich, particularly where the sample analyzed contains more than one element.
In the 1960's, flame atomic absorption spectroscopy (FAAS) instruments came increasingly into use. These FAAS instruments employ a separate excitation source, such as a hollow cathode lamp, for each element being analyzed in a sample, in addition to the flame and the monochromator used in an FAE instrument. In flame atomic absorptin (AA) spectroscopy, the atomization source (i.e., the flame) functions primarily to dissociate a sample into its constituent atoms and to leave the latter in their lowest energy state, i.e., the ground state of energy level. It is the function of the separate excitation source in AA spectroscopy to excite some of these free atoms in the ground state to a higher state of energy level. In so doing, these atoms absorb some of the excitation source radiation, and the fraction absorbed, relative to when there are no atoms of the element analyzed for present in the atomization source, is indicative of the concentration of that element in the sample. In an AA instrument, the resultant spectra are unambiguous and simple since each element absorbs best at its characteristic wavelength. The signal observed at this characteristic wavelength is indicative of the concentration of that element in the sample. In an AA instrument, however, the radiation from the separate excitation source, the atomization source and the detector are all required to be mounted along the same axis. Because of this constraint, it is extremely difficult to design any multichannel AA instrument for multielement analysis. Consequently, multielement AA analyses are carried out sequentially on single channel AA instruments.
No such constraint exists in atomic fluorescence (AF) spectroscopy. In contrast to AA spectroscopy, in AF spectroscopy the excitation source can be mounted anywhere off the atomization source-detector axis. Most AF spectroscopic instruments arrange the excitation source and the detector at right angles to each other and in a horizontal plane when viewing the isotropically emitted fluorescence radiation from the analyte in the atomization source. Consequently, designing a multichannel AF instrument for multielement analysis is inherently simpler than with AA. Furthermore, it is a characteristic of AF that the AF spectra are simple, as in AA.
During the 1970's, a promising new atomization source emerged--the inductively coupled plasma (ICP). A plasma is defined as a luminous gas, a significant fraction of whose atoms or molecules is ionized. Plasmas therefore are considered to be gaseous conductors. As such, plasmas readily interact with magnetic fields, making it possible to couple a plasma to a high frequency power source. The emergence of the inductively coupled plasma (ICP) led to the widespread use of ICP-Atomic Emission Spectroscopy (AES) systems, particularly in the simultaneous multielement analysis (SMA) for many trace elements. For unlike in AA and in AF spectroscopy, in an ICP-AES system, no separate excitation source for each element being analyzed is required. Consequently, SMA can be done in a relatively simple manner.
An ICP-AES system, however, suffers from a serious disadvantage and that is the problem of spectral line interference. This problem of spectral line interference is particularly severe when analyzing a sample for traces of metals in the presence of other metals such as tungsten, cerium, uranium, iron, vanadium and the like. It has been found that these and many of the transition series metals as well as all of the lanthanide series metals are spectral-line rich in the 220-420 nm wavelength region commonly employed in a typical ICP-AES analysis. As a consequence, any ICP-AES system must include a high resolution spectrometer. Nonetheless, an ICP-AES analysis of a sample containing a number of metals at concentrations above their respective detection levels still exhibits numerous instances of overlap of the emission lines of the elements present in the sample. The analyst then has to unravel which fraction, if any, of the total measured atomic emission signal in each channel is from an intended element and which fraction (or fractions) is (or are) from an interfering element (or elements). Slit changes and the use of computers to help disentangle the overlay are required to produce reliable and accurate results. Furthermore, the bandwidth of the atomic emission line in the wings in ICPs (and also in flames) can be 0.5 Angstroms or more. Thus, the designing of a spectrometer with a resolution better than 0.5 Angstroms, while reducing the number of instances of spectral line overlap interferences, would still not eliminate them. The problem of spectral line overlap interference, therefore, is and remains a fundamental limitation to the employment of ICP-AES systems.
The problem of spectral line overlap interference has been effectively met by the provision of an inductively coupled plasma atomic fluorescence spectrometer (ICP-AFS) system disclosed in copending application Ser. No. 152,387, Demers et al, filed May 22, 1980. The ICP-AFS system of application Ser. No. 152,387 comprises a plasma source for atomizing a dispersed sample and directed along a central axis, a plurality of replaceably mounted optical stations surrounding the central axis, each including an energizing illuminator and a fluorescence detector focused at the same region of the plasma stream, and a readout system for identifying the unknown samples. The fluorescence detector includes an optical interference filter matched to the characteristic radiation of its associated energizing illuminator, and the readout system incorporates multiplexing and intermittent modulation of the energizing illuminators.
The ICP-AFS system of application Ser. No. 152,387, effective as it is in substantially eliminating spectral line overlap interference, nevertheless exhibits some shortcomings. For one, the physical size of the optical stations places a limit on the number of channels that can be built into the system, thus limiting the number of elements that can be analyzed thereby. For another, the required presence and structure of certain of the components adds to the cost and complexity of the system. These include: use of a specific optical interference filter and expensive hollow cathode lamps and photomultiplier tubes for each element being analyzed. For still another, adjustment of the optical stations for optimum observation heights is awkward in that it requires the loosening of screws, followed by tilting of the particular station about a pivot axis and then retightening the loosened screws. For still another, since the plasma source has been operated at a lower forward rf power, its operation required careful attention lest it became extinguished upon the introduction of sample aerosol into it, with some of the elements not being efficiently volatilized, resulting in some matrix interferences.